Release 8 of the Third Generation Partnership Project (3GPP) Long-Term Evolution (LTE) standard describes piggybacking uplink control information (UCI) on a physical uplink shared channel (PUSCH). The channel quality indicator/pre-coding matrix indicator (CQI/PMI) resources are placed at the beginning of uplink shared channel (UL-SCH) data resources and mapped sequentially to all single-carrier frequency division multiple access (SC-FDMA) symbols on one subcarrier before continuing on the next subcarrier. The UL-SCH data is rate-matched around the CQI/PMI data. The HARQ-ACK resources are mapped to SC-FDMA symbols by puncturing the PUSCH data resource elements (REs). Reducing the PUSCH REs punctured by the HARQ-ACK symbols would, therefore, improve the PUSCH performance.
In light of the above, Release 8 provides a 2-bit downlink assignment Index (DAI) in downlink control information (DCI) format 0/4, VDAIUL, which is used to indicate total number of downlink (DL) assignments in a bundling window. Assuming the bundling window size is M, only VDAIUL HARQ-ACK bits, rather than M bits, need to be fed back to a transmitting device, e.g., an enhanced node base station (eNB), if PUSCH transmission is adjusted based on a detected PDCCH with DCI format 0/4. Thus, (M−VDAIUL) useless HARQ-ACK bits, corresponding to DL subframes that were not scheduled by the eNB, are reduced.
Release 10 of the LTE standard (Rel-10) introduces carrier aggregation, in which more than one component carrier (CC) may be used for data transmissions. In a Release 10 time division duplexing (TDD) system, the HARQ-ACK codebook size, in case of piggybacking on PUSCH, is determined by the number of CCs, their configured transmission mode, and number of downlink subframes in bundled window. For TDD UL-DL configurations 1-6, and when PUCCH format 3 is configured for transmission of HARQ-ACK, the HARQ-ACK codebook size is determined by:nHARQ=BcDL(C+c2),  (1)
where C is the number of configured CCs, C2 is the number of CCs configured with a multiple-input, multiple-output (MIMO) transmission mode that enables reception of two transport blocks; BcDL is the number of downlink subframes for which UE needs to feedback HARQ-ACK bits for the cth serving cell. For TDD UL-DL configuration 1, 2, 3, 4, and 6, the UEs will assume BcDL on PUSCH subframe n as:BcDL=WDAIUL,  (2)
where WDAIUL is determined by the DAI in DCI format 0/4 according to the following table:
TABLE 1DAIMSB, LSBWDAIUL0, 010, 121, 031, 14
The DAI may be communicated in a subframe that has a predetermined association with subframe n for each serving cell. For example, the DAI may be communicated in subframe n−k′, where k′ is defined in the following table:
TABLE 2TDD UL/DLsubframe number nConfiguration012345678916464244344444454677577
Since the TDD UL-DL configuration of each serving cell is always identical in Rel-10 and WDAIUL is definitely no larger than the bundling window size, the HARQ-ACK codebook size determined by WDAIUL is always equal to minimum HARQ-ACK bits number and is the best tradeoff between HARQ-ACK overhead and performance.
In Release 11 of the 3GPP LTE standard, interband CA of TDD with CCs having different UL-DL configurations for each serving cell is supported. Having different UL-DL configurations in the different serving cells may result in different HARQ-ACK bundling windows. Therefore, the UL grant based HARQ-ACK codebook size determination in previous releases may not effectively reduce the HARQ-ACK overhead.